Hollow rack shaft

ABSTRACT

A workpiece for a hollow rack shaft is a substantially rectangular plate. The opposite two sides of this workpiece are respectively provided with a complementary profile composed of the continuation of a convex portion and a concave portion and when the workpiece is cylindrically formed, the profiles are mutually engaged. As the width of a part the width of which is the largest in the convex portion is larger than that of a part the width of which is the smallest in the concave portion, the two sides are prevented from being detached after engagement. Further, the opposite convex portion and concave portion are mutually caulked and hereby, mutual gap is removed.

[0001] This application is based on applications No. 2000-269725, No.2000-269915 and No. 2000-236982 field in Japan, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a hollow rack shaft,particularly relates to a hollow rack shaft used for a steering systemof an automobile.

[0004] 2. Description of the Related Arts

[0005] A hollow rack shaft used for a steering system of an automobileand its manufacturing method are disclosed in Japanese Laid-Open PatentsNo. Hei 11-180318 and No. Hei 11-278287 and others. In any manufacturingmethod, rack teeth are formed at the bottom of a U-shaped plate bent ina gutter-like shape, and afterward, the plate is formed cylindrically bybutting two open legs of the U shape. The butt portion of the cylinderis normally welded to acquire higher strength except a case that thereis no problem on the strength and this cylinder is used as a hollow rackshaft.

[0006]FIGS. 1a and 1 b are a top view and a sectional view showing aworkpiece for manufacturing the above shaft. FIGS. 2a to 6 c show theforming process from the material to the hollow rack shaft in thismanufacturing method. These drawings accompanied by a, b and c arerespectively equivalent to a front view of the rack shaft (theworkpiece), a D-D sectional view of the part in which rack teeth areformed, and an E-E sectional view of the part in which no rack tooth isformed.

[0007] The workpiece of a substantially rectangular plate shown in FIGS.1a and 1 b, is bent into a gutter-like shape having a U-shaped part witha flat bottom in an area A1, and having a U-shaped part with asemicircular bottom in an area A2 on both sides of A1 as shown in FIGS.2a to 2 c. After a row of rack teeth is formed at the flat bottom of thegutter-shaped workpiece as shown in FIGS. 3a to 3 c, the ends of twoopen legs of the gutter-shaped workpiece are bent inside so that theyare joined as shown in FIGS. 4a to 4 c. The section of the part in thearea A2 becomes circular by this bending In most of cases, after theends of these legs are mutually welded at butt portion as shown in FIGS.5a to 5 c, the workpiece becomes a hollow rack shaft shown in FIGS. 6ato 6 c after finishing process.

[0008] Generally, welding has the cooling contraction problem whichcaused by the solidification and the contraction of melted and expandedmaterial at cooling. Internal stress is caused by the contractionthrough cooling of welded part, and distortion is also caused in thewhole workpiece.

[0009]FIG. 7 exaggeratingly shows the hollow rack shaft deformed inwhole by the contraction through cooling of the welded part 5. As thebutt-welded part 5 is located on the reverse side to the rack teeth, thewarp that the contracted welded part (a weld bead) 5 is on the insideand the row of rack teeth is on the outside is caused.

[0010] At this time, as the row of rack teeth is also influenced by thedeformation, the rack teeth are out of the standard of teeth profile andcannot be substantially used for a rack shaft as they are. That is, asthe rack shaft also warps in the area A1 having the rack teeth and theprecision of the teeth profile is deteriorated by this effect, the rackshaft cannot be normally engaged with a pinion.

[0011] Such deviation in the precision of the teeth profile of the rackcan be rectified by finishing through grinding in another process.However, as the merit that plastic working has excellent productivity islost, it is not a good policy to insert grinding process in order torectify the teeth profile in the manufacturing process on the hollowrack shaft. Further, as welding requires the process to remove the weldbead, the number of processes and the cost for manufacturing the hollowrack shaft are increased.

[0012] Further, as the rack teeth are designed as thin as possible inconsideration of the lightening, it cannot be allowed in view of thestrength that the teeth are further ground by grinding.

[0013] The hollow rack shaft has further another problem. As a pinioncoupled to a steering shaft is engaged with the rack teeth and steeringforce is transmitted from the pinion, large bending stress acts upon thededendum of the rack teeth and high contact pressure acts upon thecontact part of the surface of the rack teeth. To endure the largebending stress and the high contact pressure, the rack teeth aregenerally hardened up to approximately the hardness Hrc 60 bycarburizing quenching or induction hardening. In the meantime, such highcontact pressure and large bending stress do not act upon the part inwhich no rack teeth are formed.

[0014] Nevertheless, a workpiece of one thick plate made of high-pricedmaterial is used to suit to the largest load as a whole, and further,the rack shaft which is made in this method has the same diameter of thearea A1 that the row of rack teeth is formed as that of the areas A2 onboth sides of the area A1. As described above, in prior art, superfluousquality is given to the area A2 which receives only small load in viewof the material and the dimension, and therefore, the manufacturedhollow rack shaft does not achieve enough lightening and it ishigh-priced.

SUMMARY OF THE INVENTION

[0015] The invention provides a hollow rack shaft and its manufacturingmethod wherein a process for a butt weld is not required, therefore, theprocess of weld beads and the rectification of the welding warp are notrequired and in addition, the strength is the same as what is welded.

[0016] Further, the invention has the object of acquiring a low-pricedand light hollow rack shaft, particularly a hollow rack shaft for asteering system and manufacturing it easily. Furthermore, the inventionhas an object of using an enough thick and high quality plate in orderto apply enough strength to rack teeth, using a thin plate made oflow-priced material for part having no rack teeth, acquiring a light andlow-cost hollow rack shaft and hereby further, acquiring the usefulhollow rack shaft having suitable quality a-d the balance of thestrength as a whole.

[0017] A workpiece for the hollow rack shaft disclosed in thisspecification is a substantially rectangular plate. The opposite twosides of this workpiece have complementary profiles each of which iscomposed of the continuation of a convex portion and a concave portion.When the workpiece is formed cylindrically, the profiles are mutuallyengaged. As the size of the widest part of the convex portion is widerthan that of the narrowest part of the concave portion, theabove-mentioned two sides are prevented from being detached afterengagement. Further, the opposite convex portion and concave portion aremutually caulked, and hereby, respective gap is filled up.

[0018] A workpiece for another hollow rack shaft disclosed in thisspecification is one plate acquired by welding a first plate for a rackteeth area for a row of rack teeth to be formed and a second plate forthe other area. For the first plate, a plate thicker than the secondplate and/or a plate made of more satisfactory material in hardenabilityare/is used.

[0019] Further another hollow rack shaft disclosed in this specificationis provided with a first area and a second area on both sides of it. Inthe first area, a row of rack teeth and a semi-cylindrical part on thereverse side are formed, the second area is fully cylindrical and on atleast one side of the second area, apart different from thesemi-cylindrical part in the first area and having a particularly smalldiameter is formed.

[0020] Other objects and advantages besides those discussed above shallbe apparent to those skilled in the art from the description of apreferred embodiment of the invention which follows. In the description,reference is made to accompanying drawings, which form a part thereof,and which illustrate an example of the invention. Such example, however,is not exhaustive of various embodiments of the invention, and thereforereference is made to the claims which follow the description fordetermining the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

[0022]FIGS. 1a and 1 b are a top view and a sectional view showing aworkpiece for manufacturing a hollow rack shaft;

[0023]FIGS. 2a to 6 c are explanatory drawings for explaining theprocess that the hollow rack shaft is manufactured based upon theworkpiece drawings accompanied by a, b and c are respectively equivalentto a front view of the rack shaft (the workpiece), a sectional view online D-D of the part in which rack teeth are formed, and a sectionalview on line E-E of the part in which the rack teeth are not formed;

[0024]FIG. 7 exaggeratedly shows the hollow rack shaft deformed in wholeby the contraction through cooling of welded parts;

[0025]FIGS. 8a to 8 c are respectively a top view showing a workpieceused in a first embodiment, a side view showing the workpiece and anenlarged view of a part c in FIG. 8a;

[0026]FIGS. 9a to 9 c show the shape of a plate 1 after a first processin the first embodiment, and are respectively a front sectional view, asectional view on line B-B in FIG. 9a and a sectional view on line C-Cin FIG. 9a;

[0027]FIGS. 10a and 10 b show the shape of a workpiece 1 after a secondprocess in the second embodiment, and are respectively a front sectionalview showing the workpiece 1 after rack teeth are formed and a sectionalview on line B-B in FIG. 10a;

[0028]FIGS. 11a to 11 d show the shape of the workpiece 1 after thethird process in the first embodiment, and are respectively a frontsectional view, a bottom view, a sectional view on line C-C and asectional view on line D-D in FIG. 11a;

[0029]FIGS. 12a to 12 c are respectively an enlarged view of thevicinity of a connection A after caulking in the first embodiment, asectional view on line B-B in FIG. 12a and a sectional view on line C-Cin FIG. 12a;

[0030]FIGS. 13a and 13 b show another example of caulking in the firstembodiment, and are respectively an enlarged top view showing a convexportion and a sectional view on line B-B;

[0031]FIGS. 14a and 14 b are respectively a top view and a front viewshowing a workpiece used in the second embodiment;

[0032]FIGS. 15a to 15 d are respectively a front sectional view showingthe workpiece 1 after the first process in the second embodiment, asectional view on line B-B, a sectional view on line C-C and a sectionalview on line D-D;

[0033]FIGS. 16a and 16 b are respectively a front sectional view showingthe workpiece 1 after the second process in the second embodiment and asectional view on line B-B;

[0034]FIGS. 17a to 17 d are respectively a front sectional view showingthe workpiece 1 after a third process in the second embodiment, asectional view on line B-B, a sectional view on line C-C and a sectionalview on line D-D;

[0035]FIGS. 18a and 18 b show a different example of the workpiece 1 inthe second embodiment, and are respectively a top view and a side viewshowing the workpiece 1;

[0036]FIGS. 19a and 19 b are respectively a top view and a side viewshowing a workpiece 1 used in a third embodiment;

[0037]FIGS. 20a to 20 d are respectively front sectional view showingthe workpiece 1 after a first process in the third embodiment, asectional view on line B-B, a sectional view on line C-C and a sectionalview on line D-D;

[0038]FIGS. 21a and 21 b are respectively a front sectional view showingthe workpiece 1 after a second process in the third embodiment and asectional view on line B-B; and

[0039]FIGS. 22a to 22 d are respectively a front sectional view showingthe workpiece 1 formed in the shape of a hollow cylinder in the thirdembodiment, a sectional view on line B-B, a sectional view on line C-Cand a sectional view on line D-D.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] First Embodiment

[0041]FIGS. 8a to 8 c are respectively a top view and a side viewshowing a workpiece used in this embodiment, and an enlarged view of apart c in FIG. 8a. A workpiece 1 is, as a whole, a substantiallyrectangular plate acquired by blanking or laser cutting from coiledmetal. The opposite two sides have complementary profiles composed ofthe continuation of a convex portion and a concave portion. As the phaseof a convex portion and a concave portion is offset by a half of pitchp, the two sides can be mutually engaged in the relation of a male and afemale. The two engaged sides of the workpiece 1 are not detachedbecause the maximum width w1 of a convex portion and a concave portionis larger than the width w2 of the neck of the convex portion and theconcave portion.

[0042] This workpiece 1 has an area A1 in which a row of rack teeth isto be formed, an area A2 in which no row of rack teeth is formed andfour transition areas X between these two areas (respectively whenviewed along the length of the plate) as shown in FIG. 8a. In thetransition area X between the area A2 and the area A1, the width of theworkpiece 1 is made narrow by d1 on one side by degrees. As describedabove, when the workpiece 1 is completed as a rack shaft, the volume ofthe workpiece in each area is balanced by adjusting the width of theworkpiece 1 in the area A1 and the area A2.

[0043]FIGS. 9a to 9 c show the contour of the plate 1 after the firstprocess and are respectively a front sectional view, a sectional view online B-B in FIG. 9a and a sectional view on line C-C in FIG. 9a.

[0044] In the first process, the workpiece 1 is bent into a gutter-likeshape by press. The area A1 in which the row of rack teeth would beformed has a U-shaped section having a flat bottom 1 c and a pair oflegs le respectively shown in FIG. 9c, and the area A2 in which no rowof rack teeth is formed has a U-shaped section having a pair of legs leand a round bottom 1 b respectively shown in FIG. 9b.

[0045] In the second process, rack teeth are formed at the flat bottomof the workpiece 1 by press. FIGS. 10a and 10 b show the shape of theworkpiece 1 after the second process and are respectively a frontsectional view showing the workpiece 1 after rack teeth are formed and asectional view on line B-B in FIG. 10a.

[0046] The row of rack teeth 1 d is formed by approaching the upper diehaving teeth equivalent to the row of rack teeth to the lower die havinga corrugated shape corresponding to the rack teeth of the upper dieunder the state that the workpiece 1 is placed between these dies. Then,the shape of the two dies is transferred on the flat bottom of theworkpiece 1.

[0047] In the third process, the workpiece 1 is formed cylindrically bybending the two legs le inside. FIGS. 1a to 11 d show the shape of theworkpiece 1 after the third process and are respectively a frontsectional view, a bottom view, a sectional view on line C-C in FIG. 11aand a sectional view on line D-D. Two legs le shown in FIGS. 10a and 10b are bent inside so that a concave portion and a convex portion aremutually engaged. As the legs le are mutually combined by the engagementof the concave portion and the convex portion, the butt welding asconventional is not required and even if a load is applied to the rackshaft, the legs 1 e are not detached.

[0048] The two legs le engage more stiffly by caulking the vicinity of aconnection A as the concave portion touches the convex portion which isengaged with it without clearance, and consequently, there is no gap.

[0049]FIGS. 12a to 12 c are respectively an enlarged view of thevicinity of the connection A which was caulked, a sectional view on lineB-B in FIG. 12a and a sectional view online C-C in FIG. 12a. In thevicinity of a boundary between opposite convex portions, a substantiallyconical indent p is made. A part q of the material of one convex portionis deformed by impressing as the part covers the other convex portion.This deformation prevents the gap between the convex portion and theconcave portion, and that the convex portion falls off in a directionshown by arrow.

[0050]FIGS. 13a and 13 b show another example of caulking and arerespectively an enlarged top view of a convex portion and a sectionalview on line B-B. In this example, the gap and the falling off of theconvex portions are prevented by making an impression p in the shape ofa distorted triangular pyramid across a boundary and causing deformationq on the boundary.

[0051] Caulking described in the above-mentioned examples is arbitraryand the above-mentioned convex portion (therefore, the concave portion)is not limited to the one shown in the drawings and if only a convexportion and a concave portion can be engaged, they may have an arbitraryshape.

[0052] Afterward, carburizing quenching or induction hardening isapplied to the row of rack teeth to give the required strength of thesurface of teeth and both ends are finished as shown in FIG. 6a to be aproduct. In a halfway process, the bend of the rack shaft can besuitably rectified if necessary.

[0053] Second Embodiment

[0054]FIGS. 14a and 14 b are respectively a top view and a front viewshowing a workpiece used in a second embodiment. The plate workpiece 1used in the second embodiment is one substantially rectangular plate asa whole acquired by welding the two plate workpieces 11 and 12 which aredifferent in the material and/or the thickness at a weld R. For theworkpiece 11, the material that enables carburizing quenching such asScr and SCM or carbon steel suitable for induction hardening can be usedand the workpiece 11 is made thicker than the workpiece 12 so that a rowof rack teeth to be formed has enough strength.

[0055] As the weld R is hardened by the heat of a weld, it is annealedto get rid of bad effect upon later forming. The workpiece 1 has an areaA1 in which the row of rack teeth is formed, an area A2 in which no rowof rack teeth is formed and a transition area X in which transition byd1 from the area A1 to the area A2 as in the first embodiment.

[0056]FIGS. 15a to 15 d are respectively a front sectional view showingthe workpiece 1 after the first process, a sectional view on line B-B, asectional view on line C-C and a sectional view on line D-D. In thefirst process, the workpiece 1 is bent into a gutter-like shape having aU-shaped section as a whole. As in the first embodiment, any section hasa pair of legs 1 e, the area A1 has the section of a flat bottom 1 c andthe area A2 has the section of a round bottom 1 b.

[0057]FIGS. 16a and 16 b are respectively a front sectional view showingthe workpiece 1 after the second process and a sectional view on lineB-B. In the second process, as in the first embodiment, a row of rackteeth 1 d is formed at the flat bottom 1 c of the workpiece 1.

[0058]FIGS. 17a to 17 d are respectively a front sectional view showingthe workpiece 1 after the third process, a sectional view on line B-B, asectional view on line C-C and a sectional view on line D-D. In thethird process, the ends of two legs le are bent inside as they arejoined. The workpiece 1 is butted cylindrically. In the case thatstrength is required for the rack shaft, the ends are welded in buttparts P and Q.

[0059] Afterward, the workpiece 1 substantially having the shape of therack shaft is finished by machining, the carburizing quenching of therow of rack teeth and the other location and finishing through grindingand is shipped as a hollow rack shaft.

[0060]FIGS. 18a and 18 b show a different example of the workpiece 1 inthe second embodiment and are respectively a top view and a side viewrespectively showing the workpiece 1. The workpiece 1 is acquired byfitting material 21 for forming a row of rack teeth in a hole ofmaterial 22 and welding them. As in the above-mentioned examples, forthe material 21, the material that enables carburizing quenching such asScr and SCM or carbon steel suitable for induction hardening can be usedand the material 21 is thickened, compared with the material 22 so thatthe row of rack teeth to be formed later has enough strength. In thisexample, only the workpiece 1 is different, and the first to thirdprocesses are substantially similar to those in the above-mentionedexamples.

[0061] In the second embodiment, as high-priced and high quality asexcellent in hardenability and enough thickness steel can be used onlyin a part and the vicinity in which the row of rack teeth is formed, thelight and low-priced hollow rack shaft can be manufactured.

[0062] Third Embodiment

[0063]FIGS. 19a and 19 b are respectively a top view and a side viewrespectively showing a workpiece 1 used in a third embodiment. Theworkpiece 1 is one substantially rectangular plate as a whole and issimilar to the workpiece in the first and second embodiments about thepoint that the workpiece 1 in the third embodiment also has an area A1in which a row of rack teeth is formed, areas A2 in which no row of rackteeth is formed and a transition area X. However, in the thirdembodiment, the area A2 further has a wide partial area B1, a partialarea B2 narrower than the area B1 by d2 on one side and a transitionarea Y.

[0064] A large load is applied to the vicinity of the row of rack teethof a rack shaft, while a very large load is not applied to a part apartfrom the row of rack teeth. Therefore, the diameter in this part of therack shaft can be much smaller than the present. Accordingly, in thethird embodiment, the diameter of a part of the completed rack shaft isreduced by narrowing the width of a part of the workpiece 1, that is,the width of the partial area B2 and the weight is reduced.

[0065]FIGS. 20a to 20 d are respectively a front sectional view showingthe workpiece 1 after the first process, a sectional view on line B-B, asectional view on line C-C and a sectional view on line D-D. In thefirst process, the workpiece 1 is bent into a gutter-like shape having aU-shaped section as a whole. As in the first and second embodiments,each section has a pair of legs le, a flat bottom 1 c in the area A1 anda round bottom 1 b in the area A2. However, as different from the firstand second embodiments, the U-shaped sections which have different sizeare formed in the partial areas B1 and B2 in the area A2. In thetransition area Y, the size of U-shaped sections varies by degrees.

[0066]FIGS. 21a and 21 b are respectively a front sectional view showingthe workpiece 1 after a second process in the third embodiment and asectional view on line B-B. In the second process, as in the first andsecond embodiments, a row of rack teeth 1 d is formed at a flat bottom 1c of the workpiece 1.

[0067] In the third process, the hollow cylindrical workpiece 1 isacquired by bending a pair of legs le so that they are joined. FIGS. 22ato 22 d are respectively a front sectional view showing the workpiece 1formed in the shape of a hollow cylinder, a sectional view on line B-B,a sectional view on line C-C and a sectional view on line D-D. Theoutside diameter in the sectional views on lines B-B and C-C is Dr andthe outside diameter in the sectional view on line D-D is Dt that isshorter than Dr. The whole is lightened without the effect on thestrength of the rack shaft by reducing the outside diameter in thepartial area B2 as described above.

[0068] Afterward, the joined parts of the workpiece 1 substantiallyhaving the shape of the rack shaft are welded if necessary, theworkpiece is finished by machining, the carburizing quenching of the rowof rack teeth and the other location and grinding and is shipped as ahollow rack shaft.

[0069] In the third embodiment, the whole is lightened without theeffect on the strength of the rack shaft by reducing the outsidediameter in the partial area B2 as described above.

[0070] Although only preferred embodiments are specifically illustratedand described herein, it will be appreciated that many modifications andvariations of the present invention are possible in light of the aboveteachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. A hollow rack shaft which is formed cylindricallyby bending a substantially rectangular plate so that the parallel twosides are joined and in a part of the surface of which a row of rackteeth along a direction of the axis is formed, wherein: said respectivesides have complementary profiles composed of the continuation of aconvex portion and a concave portion so that the sides are engaged whenthey are joined.
 2. A hollow rack shaft according to claim 1, wherein:the width of a part having the largest width of said convex portion islarger than the width of a part having the smallest width of saidconcave portion; and hereby, said two sides are prevented from beingdetached.
 3. A hollow rack shaft according to claim 1, wherein: saidconvex portion and said concave portion mutually opposite are caulked;and hereby, mutual gap is removed.
 4. A method of manufacturing a hollowrack shaft which is formed cylindrically by bending the opposite sidesof a substantially rectangular plate so that the sides are joined and ina part of the surface of which a row of rack teeth along a directionof-the axis is formed, wherein: a complementary profile composed of thecontinuation of a convex portion and a concave portion is provided tosaid respective sides; and the respective sides are joined owing to thisprofile when they are confronted so that they are not detached.
 5. Amethod of manufacturing a hollow rack shaft according to claim 4,wherein: further, said convex portion and/or said concave portion ofsaid joined sides are/is caulked to deform the boundary.
 6. A hollowrack shaft which is formed cylindrically by bending a substantiallyrectangular plate so that the parallel two sides are joined and in apart of the surface of which a row of rack teeth along a direction ofthe axis is formed, wherein: said plate is one plate acquired by weldinga first plate for a rack teeth area for said row of rack teeth is to beformed and a second plate for an area except it.
 7. A hollow rack shaftaccording to claim 6, wherein: said first plate is thicker than saidsecond plate.
 8. A hollow rack shaft according to claim 6, wherein: saidfirst plate is made of material more satisfactory in hardenability thanthat of said second plate.
 9. A method of manufacturing a hollow rackshaft provided a row of rack teeth along a direction of the axis in apart of the surface for forming a substantially rectangular platecylindrically by bending it so that the parallel two sides are joined,wherein: said plate is one plate acquired by welding a first plate for arack teeth area for said row of rack teeth to be formed and a secondplate for an area except it.
 10. A method of manufacturing a hollow rackshaft according to claim 9, wherein: said first plate is thicker thansaid second plate.
 11. A method of manufacturing a hollow rack shaftaccording to claim 9, wherein: said first plate is made of material moresatisfactory in hardenability than that of said second plate.
 12. Ahollow rack shaft which is formed cylindrically by bending a plate sothat the two sides of the plate are joined and in a part of the surfaceof which a row of rack teeth along a direction of the axis is formed,wherein: said rack shaft is provided with a first area and second areason both sides of it; in said first area, the row of rack teeth and asemi-cylindrical part on the reverse side are formed; and in said twosecond areas, a complete cylindrical part is formed and at least one ofthe second areas has a diameter different from the diameter of thesemi-cylindrical part in said first area.
 13. A hollow rack shaftaccording to claim 12, wherein: the diameter of at least one of saidsecond areas is smaller than the diameter of said first area.
 14. Amethod of manufacturing a hollow rack shaft which is provide with afirst area and a second area on both sides of it, in which in said firstarea, a row of rack teeth and a semi-cylindrical part on the reverseside are formed and in which in said two second areas, a completecylindrical part is formed and at least one of the second areas has adiameter different from the diameter of the semi-cylindrical part insaid first area, wherein: a plate provided with a part with first widthhaving width corresponding to the first area for said row of rack teethto be formed and a part with second width corresponding to said secondarea and having narrower width than said part with the first width isplastically deformed cylindrically.